[Nats] set event.module and event.dataset

[mtojek]

What does this PR do?

This PR adds event.module and event.dataset to the integration.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• If I'm introducing a new feature, I have modified the Kibana version constraint in my package's manifest.yml file to point to the latest Elastic stack release (e.g. ^7.13.0).

[elasticmachine]

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Build stats

• Build Cause: Pull request #1229 updated

• Start Time: 2021-06-29T08:16:43.840+0000

• Duration: 21 min 42 sec

• Commit: a40614d

Test stats 🧪

Test	Results
Failed	0
Passed	49
Skipped	0
Total	49

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[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)

[ChrsMark]

lgtm